It is known that the organs of equipment which undergo repeated impact with liquids during functioning, are subject to a slow but continuous erosion destined to jeopardize their functionality and performances after a certain period of operation.
This phenomenon is particularly evident and significant, for example, in vapour turbines whose components are subject to marked wear when specific precautionary measures are not adopted.
Specifically in vapour turbines, the condensation pressure values must be as low as possible in order to obtain the highest outlet power in simple and combined cycles.
Under these operating conditions, the low pressure rotor blades are subjected to different chemical and physical stress and therefore undergo erosion processes due both to the presence of numerous water particles in the vapour flow and also to the high peak rates of the blades.
The erosion phenomena of vapour turbine components, which occur as a result of repeated impact with liquids under prolonged operating conditions, have already been the subject of studies and are documented in Wear, M. Lesser 1995, 28–34.
In order to avoid the drawbacks due to these erosion phenomena, attempts were made to solve the problem, from the design point of view, by increasing the axial spacing between the stator and rotor or by extracting the humidity between the rows of blades through holes or air gaps situated on the blades of the stator.
These remedies did not prove to be particularly suitable for solving the problem, as they cause a reduction in the performances of the turbine.
Attempts were then made to prolong the average operating life of the turbine blades, by studying new coating materials which are capable of reducing the erosion rate of the metals caused by impact liquid separation (F. J. Heymann, ASM Handbook Vol. 18, page 221).
Improvements in this field have so far been reached by resorting to specific treatment on the metal surface of the blades, such as induction or local flame hardening, by means of stellite plate brazing or with tool steels, or by means of hard coatings applied by welding.
In order to evaluate the resistance to erosion, the coating materials of the known art have been subdivided, approximately, into two groups, that of carbides and that of metallic materials among which Stellite 6, according to what is already described in literature for example in the publication “Erosion-resistant Coating for Low-Pressure Steam Turbine Blades, Euromat '99”.
Ionic nitriding with PVD coating using titanium nitride and chromium or zirconium nitride were selected for the surface treatment.
The blades subjected to ionic nitriding treatment followed by two subsequent PVD coatings were made up of a layer of titanium nitride followed by a coating of zirconium nitride or chromium nitride.
All the PVD coatings had a thickness of about 3–4 μm. The coating tests showed a coating discontinuity of the models and the behaviour was considered unsatisfactory.
A SEM test revealed that the PVD coating was not substantially capable of opposing impact erosion whereas the nitride layer was subject to lesions as a result of micro-fractures together with the foil nitrides present in the structure.
Blades with metallic coatings were then tested with HVOF (Triballoy 800).
The performances of the Triballoy 800 alloy, as coating material against erosion from liquids, proved to be inadequate.
From the indications obtained in the tests effected, it can in fact be held that these metal alloy coatings are not even as effective, in limiting erosion phenomena, as uncoated surfaces of the base material.
This behaviour on the part of the Triballoy 800 alloy is verified both by the results of the adhesion tests (all the coatings tested did not pass this test) and also through SEM micrographic observation which revealed the presence of numerous micro-fractures in the coating layer. The microstructure of these coatings, in fact, has a high oxide content and a marked porosity which make it unsuitable for resisting erosion by liquids.
Blades with metallic coatings (Stellite 6) were then tested with HVOF.
Although stellite alloys are known as being a material suitable for coating, they show all their limits when applied by means of HVOF. Micrographic analysis, in fact, demonstrates that low content particles are also enveloped in a film of oxide.
This fact is also confirmed by the surface morphology revealed by means of SEM, which shows a detachment or ungluing of the material specifically along these particles.
Blades treated with coatings with HVOF and SD-Gun TM carbides were then tested.
The results obtained with these types of coatings are in some cases comparable to or better than those obtained with the hardened base material (WC-1OCo-4CrSD-Gun TM and 88 WC-12Co HVOF).
The cases in which an unsatisfactory behaviour is verified can be explained by the reduced adhesion of the coating and through the known intrinsic fragility (due to the presence of chromium carbides).
Vice versa, the coatings of the known art which provide better results are those made of tungsten carbides with a cobalt or chromium-cobalt matrix, depending on the coating process used.
Coatings which have a good resistance to erosion are characterized by a detachment of the material on a small portion of the sample whereas this phenomenon is extended to a much larger surface of the materials whose resistance properties are considered unsatisfactory.
This different behaviour can be explained by considering the surface morphology.
When the layer of surface coating starts losing its conformation following the loss of material, the liquid/solid interaction is particularly complex. In this situation, the impulse or impact pressures which trigger the erosion phenomenon, are greatly influenced by the point in which there is initial contact with the drops which fall on a crest (slope), developing lower local pressures with respect to the drops which fall into a crater.
In the case of base materials, the low resistance effected by the surface makes the removal of the material almost completely uniform along the whole area involved in the test.
The unsatisfactory behaviour of most of the coatings of the known art can be explained by the reduced adhesion of the coating on the metallic substrate and the well known intrinsic fragility (due to the presence of chromium carbides).
Vice versa, the coatings of the art which provide improved results are those consisting of tungsten carbides with a cobalt, chromium-cobalt matrix, depending on the use of the coating process.
In general, the performances of the coatings with HVOF improve with an increase in the content of tungsten carbide. The micrographic morphology of the 88WC-12Co coating is, in fact, more homogeneous with respect to that of 83WC-17Co. On the other hand, the difference in performance of the same material (WC10Co-4Cr), applied by means of SD-GunTM or HVOF is quite marked. The results of the former are encouraging, whereas those of the latter are unsatisfactory.
This confirms that at present the spraying process has a significant importance in obtaining certain performances of the coating.
The thermal treatment of the known art for increasing the hardness, however, has as yet shown a reduced increase in resistance to erosion due to an excessive fragility.